The justices must decide whether the California Supreme Court ruled correctly in allowing the jury to consider allegations made in a police report as reliable evidence, or whether their inclusion in the murder trial violated the defendant's right to confront his accusers.

The case, Giles v. California, is emerging as a major test of how state and federal judges are to enforce the Sixth Amendment's confrontation clause.

If the high court sides with the defendant, the decision could make it harder to win convictions in certain types of murder, domestic-abuse, and child-molestation cases. If the majority justices embrace a less restrictive view of the confrontation clause, it could raise the possibility of defendants being tried and convicted on unsworn allegations that they are powerless to refute.

Other analysts say it holds monumental implications for the Bill of Rights. "This is about First Principles," says Joseph diGenova, a Washington lawyer and former US attorney in the District of Columbia.

"I can't conceive of how the admissibility of this statement by this dead declarant will be upheld by the Supreme Court," he told a recent press briefing.

The confrontation clause is designed to foster a search for truth in a trial by guaranteeing every defendant the right to cross-examine his accusers. It helps a jury establish the truth from mere allegations.

The courts have recognized that in some instances, so-called hearsay evidence may be admitted when a witness is unavailable to testify. But in a major 2004 ruling, the US Supreme Court reinvigorated confrontation-clause protections, instructing the lower courts to reject hearsay exceptions in all but a few instances.

The current case involves a California man, Dwayne Giles, who shot and killed his former girlfriend, Brenda Avie. At trial, Mr. Giles testified that Ms. Avie had a history of violence and had threatened to kill both him and his new girlfriend.

When Avie showed up uninvited at his grandmother's house, Giles told the jury that he shot her because he thought she had a gun and was about to shoot him.

The prosecutors at Giles's murder trial portrayed Giles as the one with a history of violence. They did it by calling a police officer to testify about a domestic-abuse report filed by Avie three weeks prior to her death.

Avie called police on Sept. 5, 2002. When the officers arrived, she told them that Giles was an abusive boyfriend who had choked her, punched her, and threatened to kill her if he ever caught her being unfaithful.

The statement was important to prosecutors because it suggested Giles had a predisposition to commit violent crimes. It offered jurors a different perspective than Giles's suggestion that he acted in self-defense.

Giles was convicted and sentenced to 50 years to life in prison.

The issue on appeal was whether the former girlfriend's statement should have been excluded from the trial. Since Avie wasn't available for cross-examination on the full meaning, context, and veracity of her statement to police, the statement should not have been presented to the jury, Giles's lawyers argued.

Prosecutors responded that since it was Giles who made Avie unavailable to testify by murdering her, he should not be permitted to benefit from his crime. They argued that by killing Avie, he forfeited his Sixth Amendment confrontation right.

The California Supreme Court agreed with the prosecutors.

In their brief to the US Supreme Court, lawyers for Giles say that, if upheld, the California decision would unravel the high court's 2004 landmark opinion that strengthened the protections of the confrontation clause.

It isn't enough that a defendant caused the unavailability of a witness at trial, Giles's lawyers say. Giles had to specifically target the victim as part of a witness-tampering scheme to prevent her testimony at trial, they say.

Since Giles did not shoot Avie to silence her, the lawyers say, his confrontation rights must remain intact.

"There are, to be sure, costs to the kind of criminal process envisioned by the Framers," Giles lawyer Marilyn Burkhardt writes in her brief. "Requiring live, in-court testimony may at times make it harder for prosecutors to win close cases. But the Sixth Amendment embodies a near absolute policy against the use of testimonial statements in the absence of an opportunity for confrontation."

Ms. Burkhardt adds: "The very purpose of the confrontation clause is to prevent the state from influencing the jury through evidence that is constitutionally unreliable because it has not been tested by cross-examination."

Lawyers with the California Attorney General's Office are urging the high court to embrace a different approach. "When a defendant has murdered the witness, he may not still insist on his 'right' to cross-examine her at his trial," writes Deputy State Solicitor General Donald DeNicola in his brief. "It makes no difference whether his motive for killing her was to make her unavailable to testify against him in court."

Mr. DeNicola adds: "It would damage the integrity of the criminal justice system to allow the killer to exclude his victim's testimony in his murder prosecution. The loss of the victim's evidence subverts the truth-finding mission of the courts."

The vast majority of child-molestation cases end with a guilty plea without any need for the child to testify in court. But a significant number of very young child victims from ages 3 to 5 are unwilling or unable to testify against their abusers, Mr. Vieth says.

Rather than watch a pedophile walk free, prosecutors have adopted special tactics to explore whether the abuser conveyed threats or messages designed to discourage the child from revealing the abuse to others.

"It may be direct – 'If you talk, I'll kill your dog or beat up Mom' – or be more broadly stated – 'Hey, you will go to foster care and I go to jail; bad things will happen to the family if you talk about it,' " Vieth says.

If the child is unable to testify, then prosecutors can ask the trial judge to rule that the pedophile forfeited his confrontation-clause rights by conveying such threats or messages. In that case, an earlier statement by the child about the abuse recorded by investigators could be admitted as evidence and presented to the jury.

But there is a problem with this approach. It will depend on how expansively or narrowly the Supreme Court views a defendant's confrontation rights, Vieth says.

"A lot of times when those messages are conveyed [by an abuser], it isn't with the intent that the child won't testify. It is with the intent that they never talk about it so there will never be an investigation, there will never be any embarrassment in the family," Vieth says.

If the Supreme Court requires proof of intent to subvert courtroom testimony, such earlier threats may become useless to prosecutors, he says.

"It will mean that some child-sex offenders will walk," Vieth says. "There is no doubt. Clearly some will walk."